NEUROMUSCULAR MECHANISM IN METRIDIUM 447 



sitive to stimulation than this very part. A shght touch with 

 a dehcate glass rod or the discharge of a small amount of dilute 

 acid on the lower portion of the column in Metridium is very- 

 sure to be followed by a vigorous contraction of the longitudinal 

 muscles of the mesenteries and a consequent withdrawal of the 

 oral disc. Our owm observations on the nervous contents of 

 the column wall as seen in sections agree very well with what 

 has been described by the Hertwigs. The ectoderm of this wall, 

 unlike that of the tentacles, exhibits neither a nervous layer nor 

 a muscular layer. Nevertheless by isolation methods sense 

 cells can be shown to be present and their connections can be 

 inferred from an experiment like the following. 



A fairly large area of the column wall of Metridium can be 

 isolated from the rest hj passing an incision around it in circu- 

 lar form. Such a plate of tissue, which in large animals may have 

 a diameter of several centimeters, when thus cut, remains organi- 

 cally attached to the animal only through its mesenteries, and 

 yet when it is touched by a glass rod or stimulated by weak acid, 

 the whole animal responds by a normal contraction. This 

 response fails when the mesenteries of the partly severed piece 

 are completely cut and the piece is allowed simply to lie in place 

 but without organic connection with the rest of the animal. We 

 therefore believe that in Metridium there are nervous connec- 

 tions from the sense cells of the ectoderm directly through the 

 supporting lamella into the mesenteries, as claimed by Havet 

 ('01, p. 400). 



In our own preparations we have found by the method de- 

 scribed in this paper that the supporting lamella of the column 

 wall contains many fibrils which course around the animal 

 more or less horizontally (figs. 5, 6). They have been seen 

 occasionally to enter the base of the ectoderm, but they are as 

 a rule limited to the supporting lamella and the deeper part of 

 the entoderm. In this last position they have already been identi- 

 fied by Wolff ('04, p. 251) in Hehactis. Where, in Metridium, the 

 mesenteries arise from the column wall, these fibrils often branch 

 and many of them pass out into the mesogloea of the mesenteries 

 (fig. 5). Their course here is approximately radial but after 



